Babesiosis caused by Babesia bigemina is a disease primarily affecting cattle, although limited other mammals may also be subject to infection by this species of Babesia. The disease is enzootic to tropical and subtropical climates where it poses severe constraints on livestock production. The risk of the disease and losses caused by it restrict movement of cattle to and from such enzootic areas thus resulting in a lost opportunity to upgrade local herds by importation of genetically superior breeds which are susceptible to the disease. Significant losses of cattle, meat, and milk production occur from Babesia bigemina induced babesiosis.
Despite many years of research relating to babesiosis, effective practical vaccines or other immunoprophylaxes against Babesia bigemina induced babesiosis are not available to the herdsman. The most common vaccination practice in use today is premunition, which is the inoculation of susceptible animals with blood infected with parasites which have been rendered less virulent. Although premunition provides good immunity against both homologous and heterologous strain challenge, it has a number of drawbacks, including: (a) induction of a carrier state which perpetuates the protozoan life cycle in the environment; (b) variation in the vaccine virulence which results in death, abortion, or clinical disease in some vaccinates; (c) contamination of the inoculant with other blood-borne infectious agents such as bovine leukosis virus, bluetongue virus, anaplasma, and theileria; (d) cumbersome and expensive production, storage and transport procedures which render vaccination impractical in many parts of the world; and (e) contamination of the vaccinates with host erythrocytes. Various experimental vaccines using inactivated Babesia parasites only provide partial protection against homologous strain challenge and poor protection against heterologous strain challenge.
The infective form of Babesia bigemina is the sporozoite which is found in the salivary gland of infected Boophilus microplus and possibly other species of Boophilus ticks. After being introduced into the tissues of the bovine host by the bite of a tick, the sporozoites enter red blood cells (erythrocytes) of the host animal. The sporozoites multiply and develop into merozoites within the erythrocytes. Infection initiates a cycle of host erythrocyte invasion and lysis which results in the clinical disease babesiosis which can often result in death of the host. Recovery from acute babesiosis is associated with immunological response including development of long lasting protective immunity against subsequent challenge.
There remains a need to overcome the known disadvantages of prior art vaccines by developing vaccines and methods for preventing or minimizing the pathological effects of Babesia bigemina induced babesiosis.